Heretofore, as a method of transferring a toner image on an electrophotographic photoreceptor (hereinafter, also referred as a photoreceptor) onto a recording material of a final image, well known is a method in which a toner image on an electrophotographic photoreceptor is directly transferred to a recording material. On the other hand, well known is an image forming method utilizing an intermediate transfer element; the method provided with another transfer process in the process of transferring a toner image from a photoreceptor to a recording material, and after a toner image being primarily transferred from a photoreceptor to an intermediate transfer element, a final image is obtained by transferring the primary transferred image on an intermediate transfer element to a recording material. Among them, the aforementioned intermediate transfer method is generally applied as an overlapping transfer method of each color toner image in a so-called full color image forming apparatus, in which a color separated original image is reproduced by means of subtractive mixing using such as black, cyan, magenta and yellow toners.
However, in either methods described above, in case of performing a copy or print of many sheets, there may be often caused toner filming on a photoreceptor or on an intermediate transfer element, and image defects in a final image due to the increased surface energy of a photoreceptor or an intermediate transfer element, which increases adhesive power thereof with a toner and decreases transfer property of a toner from a photoreceptor to an intermediate transfer element or from an intermediate transfer element to a recording material. Specifically in an image forming method utilizing an intermediate transfer element, decreased transfer property deteriorates the final image quality significantly because the method contains two transfer processes of which one process is a primary transfer means to primarily transfer a toner image from a photoreceptor to an intermediate transfer element and the other process is a secondary transfer means to transfer a toner image from an intermediate transfer element to a recording material.
That is, decrease of transfer property in an image forming method utilizing an intermediate transfer element easily causes so-called hollow characters, in which a part of a toner image is not transferred, and scattered characters.
To improve transfer property, to prevent toner filming or to improve insufficient cleaning, which are responsible for hollow characters and scattered characters, techniques in which fine particles are included in the surface of a photoreceptor to provide roughness on the surface so as to decrease adhesive power of a photoreceptor with a toner to improve transfer property, or to reduce friction power with a blade, having been studied. For example, JP-A 5-181291 (JP-A refers to a Japanese Patent Publication Open to Public Inspection) discloses that alkylsilsesquioxane resin fine particles are included in a photosensitive layer. However, since alkylsilsesquioxane resin fine particles have hygroscopic property to enhance surface wettability of a photoreceptor, that is, to increase surface energy, there causes such problem that transfer property is liable to be depressed. Further, JP-A 63-56658 discloses an electrophotographic photoreceptor including fluorine-contained resin powder to make the surface of a photoreceptor have a low surface energy. However, sufficient surface strength could not be obtained by fluorine-contained resin powder and a problem caused that streak defects were easily produced due to flows on the surface of a photoreceptor.
Further, a technique is disclosed in which a solid lubricant is supplied to an intermediate transfer element and a surface energy of the intermediate transfer element is reduced to improve transfer property of an intermediate transfer element. For example, there are techniques described in such as JP-A 6-337598, 6-332324 and 7-271142. However, it has been found that only to control the surface of an intermediate transfer element is still not sufficient to improve the total transfer property of an image forming method which utilizes an intermediate transfer element provided with two transfer processes, in particular further improvement is required for forming a copy image under high temperature and high humidity or during prolonged operation.
That is, it has been found that in an image forming method utilizing an intermediate transfer element, it is necessary to decrease surface energy of both of a photoreceptor and an intermediate transfer element in suitable balance and to improve the total transfer property of both primary and secondary transfer processes.